<script language="JavaScript">
bg = new Array(3);
bg[0] = '{get(cslide_pic1)}'
bg[1] = '{get(cslide_pic2)}'
bg[2] = '{get(cslide_pic3)}'

index = Math.floor(Math.random() * bg.length);
document.write("<img src="+bg[index]+" />");
</script>